The invention generally relates to systems and methods for fatigue or stress testing to fracture one or more implantable medical devices such as the mesh grid tubes used for implantable medical stents. More specifically, the invention relates to systems and methods for stressing a stent or mesh tube to fracture or break which provides valuable data for estimating the usable life of such devices.
Modern medical procedures routinely include the employment of implantable devices into a patient's vascular system to perform various therapeutic functions. Prosthetic vascular implants, such as heart-valves, stents, grafts, mesh tubes, and stent-grafts used for human implantation are subjected to the continuous fluctuating stress of blood pressure. As an example, tubular mesh samples or stents are often inserted in an artery of a patient to maintain a flow lumen through the artery at a location that had previously been at least partially blocked or occluded. It is therefore necessary to test such implants to prove their durability over a lifetime of exposure to pulsatile blood pressure. Ideally such stents, mesh samples, or other vascular prostheses, are able to withstand the physiological dynamics that occur within the vessel or organ in which they are emplaced. For instance, in the abdominal aorta, blood pressure in the average healthy subject is 120 mm Hg/80 mm Hg, i.e. the blood pressure varies by 40 mm Hg for every pulse. Compliance of a healthy aorta can be of the order of 20-25% per 100 mm Hg so that a change in vasculature diameter of 8 to 10% can be expected at every heartbeat. In order to simulate such a change in diameter, testers employ a pulse pressure between 80 mm Hg and 100 mm Hg. The vessel (artery, vein, etc) in the human body where an implantable medical device is to be implanted is referred to as the target human vessel.
One widely used methodology of testing implantable medical devices is referred to as “testing-to-success”. Typically, in testing-to-success, vascular implants are tested for 400,000,000 cycles which represent approximately 10 years of implantation life at a heart rate of 80 beats per minute. Testing-to-success is indicative of the durability of the stent under physiological conditions of systolic/diastolic pressures encountered in accelerated radial pulsatile durability testing. However, testing-to-success does not predict the endurance limit or fatigue life of the stent or other implantable device, i.e., there is no way to know under what conditions, including conditions that may exceed physiological parameters, the stent or stent graft would fail. The endurance limit, also referred to as the fatigue limit, is a well-known concept in stress testing and materials science.
To address this weakness in the “testing-to-success” methodology, new regulations (FDA, ASTM, ISO) are being developed that outline test requirements that are now concerned with predicting fatigue limits of the stent or stent graft and require stent manufacturers to test their products under a ‘testing to failure’ or ‘test to fracture’ regime, so that stents and stent grafts may be tested up to their endurance limit. An alternative method that is being rapidly pursued and evaluated is a “fatigue-to-fracture” approach. This testing methodology involves a combination of finite element analysis modeling and in vitro testing to assess the durability of stents or other implantable through established fracture mechanics techniques, such as use of a Wöhler curve, to identify the endurance/fatigue limit of a device. These testing guidelines and standards are still under development, i.e. ASTM F04.30.06.
Knowing when and where fracture, secondary fracture, or other failure, of the stent, mesh tubes, or other prosthesis, is most likely to occur under a variety of simulations is ideal to device development. Manufacturers can then use this information to redesign their product with the knowledge gained by fatigue to fracture analysis. The occurrence of the foregoing described fracture or other failure is referred to herein as a failure event. Providing a stent, or other prosthesis, of suitable strength and durability for lasting implantation into a patient, to minimize the likelihood of failure is desirable. Determining the fatigue and endurance limits of the stent, or other prosthesis, helps accomplish the provision of a suitable stent, or other prosthesis.